This invention relates generally to equipment and method for testing pavement mixes, commonly called asphalt, for the potential for water damage. More specifically, this relates to using water in a pressurized chamber to simulate the action of water being pressed into and pulled out of the wet pavement by tires on asphalt paving on a roadway.
Paving mixes known as asphalt consist of approximately 95 percent aggregates and five percent liquid binder. The mixture should be designed to create the best possible bond between the liquid binder and the aggregate. Moisture can penetrate asphalt, which causes an adhesive failure between the binder and the aggregate or water can soften or emulsify the binder film. In either case, water can reduce the strength of the mixture of the asphalt. When the liquid binder is stripped from the asphalt, the aggregate can become scattered or lost. Loss of strength in mixtures can result in pot holes in the pavement or cracking or rutting.
It is well understood that moisture can strip the binder from the aggregates, resulting in a form of failure called xe2x80x9cstrippingxe2x80x9d of an asphalt pavement. The cause of moisture damage to asphalt is multifactorial. First, the type of aggregates used in the mixture affect the susceptibility of the mixture with the binder to moisture damage. For example, residual clay left in aggregates after washing can cause a serious problem. Clay expands when it absorbs the water and creates a barrier between the aggregates and the binder effectively reducing the adhesion or cohesion of the bond between the binder and the aggregates. The composition of the binder also plays an important role in the resistance of the asphalt to moisture damage. The binder viscosity is affected by the mixing temperature in the plant and the ingredients of the binder, such as polymers and rubbers, can also affect the ability of the binder to coat the aggregate surface and to keep the aggregates bound. The binder emulsification has to be controlled to give strength and resistance to moisture for the asphalt. The aggregates should be dried carefully at the plant. Typically, there should be no more than 0.5% moisture retained in the plant produced mix. If water remains in the aggregates, then, during the actual laying of the pavement, steam can be produced which causes stripping of the binder from the aggregate. Controlling the amount of field compaction is necessary to reduce the amount of external water that can penetrate the pavement. A compact pavement with the optimum density and lack of air voids will reduce water permeability, hence reduce the possibility of water damage. However, compaction can be carried too far, which can cause rutting due to mixture instability. If, during construction, there are layers of asphalt mixtures, water can be trapped between the pavement layers. Segregation which is caused by aggregates gradation change when laying down the pavement can have a detrimental affect on asphalt pavement and induce moisture damage. Proper drainage is critical in design and construction of asphalt pavement.
It is apparent, from the above discussion, that susceptibility to water damage or stripping to an asphalt pavement can arise from many sources. Even an ideal mixture of binder and aggregate properly processed or installed can still be susceptible to water damage. Evaluation of moisture susceptibility has become an important part of volumetric design procedure and pavement construction quality control. However, the most important test to determine the susceptibility of water damage for an asphalt mixture requires testing the compacted asphalt mixture in a way that will predict susceptibility of that compacted mixture to water damage.
The most widely used test for moisture sensitivity is covered under American Association of State Highway and Transportation Official (AASHTO) specification T283 and American Society of Testing and Materials (ASTM) D4867. In both of these methods 2 sets of samples of asphalt of approximately 6 inch in diameter by 4 inch thickness are compacted in laboratory compaction equipment. The mixture can be prepared in the laboratory or can be obtained from a field site. One set is saturated with water and is kept in a temperature controlled water bath at 60xc2x0 C. for 17 to 24 hours. The control set is kept at room temperature (25xc2x0 C.). In some situations (cold climates), the sample set is also kept at 0xc2x0 C. for extended time to provide a climatic cycle of cold to hot. Both conditioned (sample) and unconditioned (control) sets are then placed in a break press and broken to determine the pressure at which the sets break apart. The ratio of unconditioned (control) to conditioned (sample) sets break pressure is then used to determine the sensitivity of the mixture to moisture damage. If this ratio {(Conditioned sample strength/Unconditioned sample strength)*100} is over 70, then the mixture passes this test and is deemed acceptable. A visual inspection of the broken conditioned sample may reveal adhesion loss and provide useful information in the inspection stage. The acceptance ratio varies and can range from 70 to 85 depending on the agency and the mixture type. Unfortunately, the reliability and repeatability of this test is very poor, the test does not simulate the true dynamics of the field conditions and the results cannot be correlated to the actual field performance.
In an attempt to create pore pressure within a compacted sample and to better emulate the actual field conditions, in 1974, Rudy Jimenez of Arizona introduced the Double Punch method. This method included a compacted sample that was held under load by a punch or a plate from top to bottom of the sample. The sample was kept under water and a sinusoidal load (5-30 psi) was applied to the sample repeatedly. Even though this method could introduce pore pressure within the sample, it still did not simulate the actual dynamics of the water movement in and out of the pavement through tire activity. Furthermore, the testing time is too long and does not correlate to field performance.
Recently, wheel rutting devices have been used to predict stripping and moisture damage. These devices use a small wheel that travels back and forth on a compacted sample that is immersed in 50 C water. Force is applied to the wheel in various amounts. Although these devices can predict the rutting rate in the pavement, the results have not been correlated to stripping or moisture damage.
Another system that has been used in research is called an Environmental Conditioning Chamber (ECS). This device was developed at Oregon State University in 1987. In this test, a sample is placed in a chamber filled with 60 C water and confining pressure of 2.5 in Hg. A conditioning direct load of 200 lbs. is applied on the sample for 0.1 sec. and then released for 0.09 sec. In this device the resilient modulus of the sample is measured before and after the loading/conditioning process. Empirical criteria is developed based on performance of known mixes to establish pass/fail limits for moisture damage. Unfortunately, this test takes 6-18 hours and so far has had poor repeatability. Also, the apparatus needed to conduct this test is extremely expensive and large for a typical laboratory application in the construction industry. This apparatus is mainly used for research and is not widely available.
Harris et al., U.S. Pat. No. 5,987,961, discloses an apparatus for testing asphalt. Rollers are driven over a pair of pavement samples placed in trays beneath the wheels. The samples are placed in trays which are in a water bath. It is controlled by a computer which continuously monitors where the pavement sample is by a displacement transducer. Terrell et al., U.S. Pat. No. 5,365,793, discloses an asphalt sample in a sealed container. A pressure differential is created across the asphalt and passes water or air or a mix through the asphalt sample by the differential pressure between the vacuum and the supply of fluid which flows through the specimen. For the Terrell device, a typical test procedure will take more than twelve hours.
Despite this earlier work it would be an advance in the art to provide an instrument that can be used during design and quality control to determine the stripping potential and moisture susceptibility of an asphalt mix. The device should simulate as far as possible the action of a tire passing over asphalt on wet pavement in which water is forced in and then drawn out of the pores in asphalt by pressure differentials created by passing of the tire. The device should be simple to operate. The cycle of testing should be relatively short in time. It should produce repeatable results.
The current invention consists of at least one sealable chamber. The test will proceed by preparing two identical compacted asphalt samples. One sample will be a test sample and one sample will be a control. The chamber can be configured to different sized samples. The test sample is placed within the chamber. Water is added to the chamber sufficient to completely cover the test sample. Water temperature can be controlled and changed to simulate environmental effects on the test sample. The air in the remaining portion of the chamber is pressurized. There is an outlet for the water in the chamber, which is forced from the chamber by the pressurized air into a reservoir. After a sufficient predetermined amount of water is forced from the pressurized chamber into the reservoir, pressure within the sealed chamber is released and allowed to return to atmospheric pressure. Pressurized water forced into the test sample during the period of increased pressure will bleed from the test sample in order to equalize pressure within the test sample and the surrounding atmosphere inside the now unpressurized chamber. Water from the reservoir will be returned to the test chamber to cover the test sample. Again, pressure will be applied to the test chamber to repeat the cycle. There are cycles of alternately pressurizing and depressurizing the test chamber. A cycle ordinarily takes around one to ten seconds. In this way, water is first forced into the test sample by the pressurization of the sealed chamber and the sealed test chamber is depressurized causing water to bleed from the test sample. As the alternate cycle of pressurizing and depressurizing continues, a certain amount of binder will bleed out of the test sample and into the water changing the color and conductivity of the water. The change in the color of the water, or its turbidity and the changes in its conductivity, can be tested through commercially available sensors. The number of the cycles of pressure will stop based on a predetermined set of criteria. These criteria can include the number of cycles, the passage of a period of time, the degree of turbidity and/or conductivity of the water. As the predetermined testing cycle is complete, the test sample can be removed from the chamber and alternatively it can be tested for damage caused by the testing cycle. This can be done in a variety of ways including testing the breaking pressure required to break the sample using a press that is commercially available for testing asphalt samples. The test sample break pressure could be compared to the control sample break pressure, thus to determine the affect the test cycles had on the test sample. The current system and its various embodiments provides the means for simulating and accelerating moisture induced damage in asphalt mixtures. Additionally, sensor capabilities are included to monitor the system functions and to measure changes in water turbidity and conductivity. The moisture damage is induced within a compacted sample by varying one or more of the following constraints such as pressure, temperature, cycle speed, liquid composition, and liquid level.